1. Field of the Invention
This invention relates to current sources which provide a constant current through variations in the supply voltage and, more particularly, to a small size, low voltage current source.
2. Description of Related Art
The function of a current source circuit is to supply a substantially constant output current, despite fluctuations in supply voltage, temperature, or other operating conditions.
A low voltage current source has been designed for use on the LT1073 series of switching regulator integrated circuits from Linear Technology Corporation (LTC).
An equivalent schematic of the LTC current source is shown in FIG. 1. PNP transistors Q4 and Q5 are shown, each having three collector terminals and each electrically identical to three separate transistors. The circuit is supplied a voltage V.sub.ss from a voltage supply terminal. The base terminals of the transistors Q4 and Q5 are connected in common so that transistor Q5 acts as a current mirror. The output currents I.sub.c on the collectors 10 and 12 of transistor Q4 are the same.
NPN transistors Q2 and Q3 have their collectors connected to collectors 10 and 12, respectively, so that current I.sub.c flows through each of transistors Q2 and Q3. The bases of transistors Q2 and Q3 are connected in common and are connected directly to the collector of transistor Q3.
Transistor Q3 is sized to have an emitter area ten times as large as the emitter area of transistor Q2. Thus, the V.sub.BE of transistor Q3 will be less than the V.sub.BE of transistor Q2 when these transistors are drawing the same current I.sub.c. In the circuit of FIG. 1, it is assumed that the V.sub.BE of transistor Q2 is about 0.7V while the V.sub.BE of transistor Q3 is about 0.64V. Thus, the emitter voltage of transistor Q3 is 60 mV.
The emitter of transistor Q3 is connected to ground through resistor R4, having a value of 15K ohms. Thus, the current I.sub.c through transistor Q3 and resistor R4 equals 60mV/15K ohms or 4 .mu.A. Since transistor Q5 is connected as a current mirror with transistor Q4, the output current I.sub.0 of transistor Q5's collector terminal 14 will be the same as I.sub.c (i.e., 4 .mu.A).
Regulation of current I.sub.0 with changes in V.sub.ss is as follows. Transistor Q1 is connected between transistors Q4 and Q2 to provide a feedback control signal to the base of transistor Q4. The collector terminal of transistor Q2 is connected to the base terminal of transistor Q1, and, therefore, the collector voltage of transistor Q2 controls the conductivity of transistor Q1, which in turn controls the conductivity of transistor Q4 and the current I.sub.c. Resistor R3, connected between the emitter of transistor Q1 and ground, reduces the loop gain, making it easier to stabilize the loop.
As the supply voltage V.sub.ss increases, the collector-to-emitter voltage of PNP transistor Q4 increases. Since transistor Q4 has a finite output impedance, this increases current I.sub.c. This increased current flows into the collector of transistor Q3 and through resistor R4. If transistor Q4's collector 12 current I.sub.c becomes greater than 60 mV/R4, the additional current will raise the IR drop across resistor R4 to greater than 60 mV, thereby raising the base-emitter voltage (V.sub.BE) of transistor Q2. Since the IR drop across resistor R4 rises linearly with an increase in I.sub.c and transistor Q2's collector current increases exponentially as its V.sub.BE increases, transistor Q2's collector current will increase more than transistor Q3's collector current I.sub.c increases. Therefore, transistor Q2's collector voltage decreases when Ic rises above 60 mV/R4. This, in turn, lowers the conductivity of transistors Q1 and Q4 and decreases I.sub.c.
Conversely, if V.sub.ss decreases, and I.sub.c becomes less than 60 mV/R4, transistor Q2's collector voltage will increase, leading to an increase in I.sub.c. Thus, the circuit of FIG. 1 uses negative feedback in order to keep I.sub.c and I.sub.0 constant regardless of changes in V.sub.ss.
A feedback system must be properly compensated to ensure that the feedback loop will not oscillate. In FIG. 1, a capacitor C1 of 40 pF is placed between the collector terminal of transistor Q2 and the collector-base terminal of transistor Q3 in order to stabilize the feedback loop.
In FIG. 2, this original feedback circuit has been redrawn to make the feedback loop easier to analyze. Note that C1 has been repositioned with a series resistor R.sub.x that is equal to the forward biased base/emitter diode resistance (r.sub.e) of transistor Q3 plus R4. The value of R.sub.x is therefore (26 mV/4 .mu.A)+15K.OMEGA.=21.5K.OMEGA.. This is the A.C. equivalent of capacitor C1 connected between transistor Q1's base and transistor Q3's collector-base.
The RC network of capacitor C1 and resistor R.sub.x stabilizes the feedback loop by creating at the base of transistor Q1 a dominant pole with a frequency of approximately 2.5 KHz. The zero (1/2.pi.R.sub.x C1) is calculated to be around 185 KHz. Creating a dominant pole at 2.5 KHz and a zero at 185 KHz ensures that the feedback loop gain will reach 0 dB prior to a phase shift of -180.degree. in the loop.
A major shortcoming of the circuit of FIG. 1 is that the die size is very large, due to the high capacitance required of its feedback capacitor C1. The die area of the capacitor could be greater than that of all the other circuit components combined, when fabricated on an integrated circuit. A smaller value of the capacitance providing equivalent or better feedback loop stabilization would lead to a significant reduction in the die area of the circuit, resulting in a major improvement. It is therefore desirable to design a substitute for the circuit of FIG. 1 which has a smaller feedback capacitor and a correspondingly reduced die size.